Microphones are regarded as the highest fidelity and best method to record acoustic instruments. Artists performing with acoustic instruments have been limited in their ability to employ external microphones due to feedback, loud bands and the requirement to stay in one place. Microphones have been tried inside of acoustic instruments with limited success due to feedback and undesirable resonances present inside of the instruments.
In a number of instances, internal microphones have been paired with under saddle or magnetic pickups with some modest success. However, the microphone is used very sparingly to avoid such things as feedback and undesirable resonances. The Duet, manufactured by L.R. Baggs Corporation of Nipomo, Calif., was one of the first successful microphone/pickup combinations. The Duet uses a crossover tuned to 1500 Hz and blended the fundamental and first harmonic range of the pickup with the upper harmonics of the microphone. This system could be played at very high sound pressure levels (SPLs) without feedback and with good fidelity.
A variety of specialized microphones are used in other applications. For example, boundary microphones are used in applications including monitoring full room sound. A boundary microphone is characterized in that it is mounted within the “pressure zone” proximate a rigid boundary and generates a signal indicative of the totality of the ambient sound reflected by the boundary. Boundary microphones are described in U.S. Pat. No. 4,361,736 to Long et al., the disclosure of which is incorporated herein by reference in its entirety. Long et al. describe that placing the diaphragm of the microphone capsule parallel to and facing the plate boundary provided by the microphone package enables the microphone to completely remove unwanted variations in the spectra of the output signal in the selected frequency range due to the cancellation and additions which would otherwise be caused by the interaction of the direct acoustical signal and the acoustical signal reflected form the proximate boundary. Long et al. explain that this is due to the fact that the microphone is operated in a mode in which the microphone senses only the variation in acoustical pressure and cannot discriminate with respect to the direction or angle of incidence of the sound. Long et al. note that it is essential that the diaphragm be shielded from all acoustic signals except those reflected from the boundary.
Another type of microphone is a noise cancelling microphone, which is a microphone designed to filter out ambient noise from the desired sound. Noise cancelling microphones typically utilize a noise cancelling capsule that is very proximity sensitive and includes at least two ports, where the front port is oriented toward the desired sound and the other port is more distant. A noise cancelling capsule's diaphragm is typically placed between the two ports; sound arriving from an ambient sound field reaches both ports more or less equally. Sound that's much closer to the front port than to the rear will make more of a pressure gradient between the front and back of the diaphragm, causing it to move more. A noise cancelling microphone capsule's proximity effect can be adjusted so that a flat frequency response is achieved for sound sources very close to the front of the microphone. Noise cancelling microphone capsules were developed specifically for telecommunications applications such as cell phones, headset microphones, etc. where rejection of background noise is paramount.
There are also other types of non-microphone systems that have been developed to amplify an acoustic instrument. Many of these are intended to act similarly to microphones and many of them amplify similar bands of frequencies from an acoustic instrument as a microphone. Particularly devices referred to as Sound Board Transducers or SBTs have been developed sometimes with the intention of replacing a microphone to amplify an instrument.
An SBT is a type of transducer device that will turn the vibration from a vibrating surface into an electrical current while making contact with that surface. In comparison, a microphone will turn a vibration in the air into an electrical signal.